The present invention relates to novel radiation-sensitive compounds, and a mixture prepared therewith, in particular a photoresist mixture. The mixture comprises a water-insoluble resinous binder which is soluble, or at least swellable, in aqueous alkaline or organic solvents, optionally an acid-activable crosslinking agent and an ester or an amide of a substituted 1,2-naphthoquinone-(2)-diazide-4-sulfonic acid as a radiation-sensitive compound. The invention further relates to a copying material which is prepared from a coating base and from the radiation-sensitive mixture. The copying material may optionally be used for producing positive or negative relief copies of a master or also for a combination thereof.
It is known that normally positive-working copying materials based on 1,2-benzoquinone- and 1,2-naphthoquinone-(2)-diazide-sulfonic acid derivatives also yield negative relief images as a result of a polarity reversal while maintaining a certain sequence of processing steps. The preparation and processing of such copying materials is described, for example, in U.S. Pat. No. 3,264,104, GB-A-2,082,339, DE-A-2,529,054, corresponding to U.S. Pat. No. 4,104,070, DE-A-3,325,022, corresponding to U.S. Pat. No. 4,581,321, and EP-A-0,212,482.
The photosensitive coating of the known copying materials is essentially composed of alkali-soluble cresol-formaldehyde novolak resins in combination with photosensitive substances such as 1,2-benzo- or 1,2-naphthoquinone-(2)-diazide derivatives. Resins and photosensitive compounds are dissolved in an organic solvent or solvent mixture and deposited in a thin coating on a coating base suitable for the particular application. Although the resin components of these resist mixtures are soluble in aqueous alkaline solutions, the photosensitive 1,2-quinonediazide compounds have a solution-inhibiting effect on the resin. On exposing the coated base to an image with actinic radiation, the photosensitive compound undergoes a structural change due to the irradiation, as a result of which the exposed regions of the coating become more soluble than the unexposed regions. Because of this solubility difference, the exposed regions of the copying layer dissolve in alkaline developer solution, while the unexposed regions remain essentially unchanged and intact so that a positive relief image of the master is produced on the coating base.
In most cases, the exposed and developed copying material is furthermore treated with an etchant, the regions of the copying layer which have not been stripped off by the developer protecting the coating base against the etchant. In this way, an etched image which corresponds in its polarity to the mask, template or other master used during exposure is produced on the coating base.
In a specific embodiment of a negative-working copying layer, for example a photoresist, based on 1,2-quinonediazide compounds, the light-affected regions of the coating are "cured" by crosslinking of the resin molecules after exposure to an image and subsequent heat treatment. The resin components are cured as a rule by a "crosslinking agent" which is incorporated in the coating and which is activated by the acid produced during the exposure of the 1,2-quinonediazide and by the heat treatment. The temperatures used in heating are below the decomposition temperature of the 1,2-quinonediazide. The heating may be carried out by irradiation, convection, by contact with heated surfaces, for example rollers, or by immersion in a heated bath of an inert liquid, for example water. The temperature may be between 100.degree. and 150.degree. C., preferably between 90.degree. and 130.degree. C.
Effective crosslinking agents are in general compounds which are easily able to form a carbonium ion under the acid and temperature conditions described. Examples thereof are the hexamethylol melamine ethers in accordance with DE-A-3,325,022, corresponding to U.S. Pat. No. 4,581,321, and also the compounds, proposed in EP-A-0,212,482, containing two or more hydroxyl or alkoxymethyl groups in aromatic molecular structures such as 1,4-bishydroxymethylbenzene or 4,4'-bismethoxymethyldiphenyl ether. 2,6-Dimethylol-p-cresol in accordance with U.S. Pat. No. 4,404,272 is also known as a crosslinking agent.
After the heat treatment, the photoresist coating is as a rule subjected to a total exposure ("flood exposure") in order to convert the still photosensitive coating regions completely into an alkali-soluble form. The flood exposure may in general be carried out under the same light source which was also used for the exposure to image.
Following the flood exposure, development is carried out with an aqueous alkaline developer solution which is normally also used in the case of a positive-working photoresist, for example aqueous solutions of sodium hydroxide, tetramethylammonium hydroxide, trimethylhydroxyethylammonium hydroxide, alkali phosphates, alkali silicates or alkali carbonates, which may contain wetting agents or small amounts of organic solvents. The development washes out the coating regions which were not affected by light in the original exposure to an image (negative copying material).
In the case of positive-working copying layers of the same coating composition in which the curing process has not been set in operation by the appropriate procedure, on the other hand, the coating regions exposed during the exposure to an image are washed out by the developer (positive copying material).
In most cases, the exposed and developed coating base is furthermore treated with an etchant, the copying layer remaining behind on the coating base having a protective function, depending on the processing method. The etchant is therefore able to act only on the coating-free regions of the coating base. An etched image, which in the case of a positive-working copying layer corresponds to the polarity of the exposure mask or, in the case of a negative-working copying layer reverses the polarity of the exposure mask, is produced in this way on the coating base.
The positive or negative relief image of the copying layer produced by the above-described processing methods on the coating base is suitable for various applications, for example as an exposure mask or as an image in the production of semi-conductor components in microelectronics, as a printing form for letterpress, gravure or lithographic printing, and also for the production of nickel rotary cylinders by electroplating.
Important criteria used to assess the suitability of a copying layer, for example a photoresist, for commercial purposes are, inter alia: the photosensitivity, the development and image contrast, the resist resolution and the adhesion of the resist to the coating base.
High photosensitivity is important for a photoresist, in particular, if it is used for applications in which several exposures are necessary, for example in the production of multiple images in a repetitive method or in those cases where light of lower intensity is used, for example in projection exposure procedures in which the light is passed through a series of lenses and monochromatic filters, as in the case of projection exposure units ("steppers") which employ monochromatic light.
The development contrast is a measure of the ability of a photoresist to transmit the dimensions of the mask reliably and precisely through the entire thickness of the coating. In the ideal case, the dimensions at the top of the coating are precisely the same dimensions as those at its bottom. A photoresist with improved contrast therefore has steeper edges.
The resist resolution relates to the ability of a photoresist system to reproduce even the finest lines and spaces of a mask used for the exposure, the exposed and developed regions having a high degree of edge steepness and edge sharpness.
In many technical fields of application, in particular in the production of semiconductor components in microelectronics, the photoresist employed has to have a particularly high resolving power if it is required to reproduce very small line widths and space widths (approx. 1 .mu.m). This property the ability to reproduce superfine dimensions in the order of magnitude of 1 .mu.m and less--is of the greatest importance for the large-scale production of integrated circuits on silicon chips and similar components. If photographic methods are used, the integration density on such a chip can be improved only by increasing the resolving power of the photoresist. The miniaturization of microprocessors and other semi-conductor components in microelectronics makes it necessary to use those methods which proceed as rapidly, reliably and simply as possible and yield reproducible results for structuring suitable substrates.
The specialist and patent literature proposes methods which make it possible to produce negative resist relief images with photoresist coatings based on 1,2-quinonediazide derivatives using a particular procedure, such as S. A. MacDonald et al., "Image Reversal: The Production of a Negative Image in a Positive Photoresist", page 114, IBM Research Disclosure, 1982; E. Alling et al., "Image Reversal of Positive Photoresist. A new Tool for Advancing Integrated Circuit Fabrication", Proceedings of the SPIE, Vol. 539, page 194, 1985; U.S. Pat. No. 4,104,070 and U.S. Pat. No. 4,576,901.
The technical implementation of these known image reversal methods is in some cases very complicated and therefore not practicable for photoresist processing in microelectronics. The quality of the resist structures is in most cases not reliably reproducible. In addition, a disadvantage is that, to carry out the reversal process, in some methods an amine gasification of the photoresist coating which can be controlled technologically only with difficulty is necessary or additives have to be incorporated in the photoresist coating which appreciably reduce the shelf life.
It is also known that the resolving power of a photoresist based on 1,2-quinonediazide derivatives is better by approx. 0.2-0.3 .mu.m in an image reversal method than in a positive procedure.
EP-A-0,212,482 proposes a method of producing negative relief image structures from a positive-working copying material which essentially contains a water-insoluble resin which is soluble in alkaline solvents, a 1,2-quinone-(2)-diazide-4-sulfonic acid ester as a photosensitive compound and a cross-linking agent which functions in the presence of acid. Due to the composition of the coating and to the course of the process in producing negative relief image structures, some of the disadvantages of the known earlier methods are eliminated, such as, for example, fewer process steps, no treatment with substances with an alkaline reaction or which form salts, no use of particularly high-energy exposure sources such as, for example, electron beams. A disadvantage is, however, that the photosensitive coating proposed in EP-A-0,212,482 is primarily suitable for exposure in the middle UV range (313-365 nm) and for i-line exposure (365 nm).
The Japanese Published Specification 27,835/88 discloses aryl esters of halogen-substituted naphthoquinonediazide sulfonic acids which can be used for photoresist materials. They are basically suitable for light of the wavelength .lambda.=365 nm and broad-band exposure with mercury high-pressure lamps. The light sensitivity for 436 nm exposure is very low and not realistic. They are suitable, in particular, for resist formulations sensitive to electron beams in which halogen-substituted compounds prove to be particularly advantageous.
Only a few useable exposure units (i-line steppers) are available for the 365 nm exposure since the technological requirements for this still have not been ideally solved. In contrast to this, g-line exposure (436 nm) is very predominantly employed at present in microlithography. The overwhelming majority of exposure units for high resolution (g-line steppers) are designed for this wavelength. There is therefore an urgent need to improve the resolving potential in the case of g-line exposure without capital-intensive installation of steppers which operate at shorter wavelength. This is possible with an image reversal method.
The solution route found for i-line exposure (365 nm) is, however, not practicable for g-line exposure (436 nm). The photosensitivity of these known resists is very low at 436 nm, which manifests itself in longer, unrealistic exposure times and the poorer development contrast and image contrast and lower microstructure resolving power associated therewith.